HER3, also known as ERBB3, is a member of the ErbB protein family. The ErbB protein family consists of four closely related transmembrane tyrosine kinase receptors, epidermal growth factor receptor (EGFR) (also known as HER1), ERBB2 (HER2), ERBB3 (HER3), and ERBB4 (HER4). Each receptor comprises an extracellular domain, an α-helical transmembrane segment and an intracellular tyrosine kinase domain. Dimerization of the receptors is an essential requirement for receptor function. Different heterodimers can form between the ErbB receptor protein family, and which heterodimers are formed and which ligands are bound determines which signaling pathways are activated.